Cnidarian and ctenophore embryos share a curious feature, exemplified by this zygote of the siphonophore Nanomia: the cell division furrow cuts through the cell like a dull knife, squeezing the cell surface inward from one side only.  The far side never seems to participate.  In both cnidarians and ctenophores the cleavage furrow initiates over a mitotic apparatus that is located just underneath the cell surface.

This unilateral cleavage furrow is interesting because the prevailing idea among cell biologists is that circumferential contraction of an equatorial belt of aligned actin filaments and myosin – the contractile ring – drives the animal cell cleavage furrow inward.  The phenomenon of unilateral furrowing challenges this notion because it’s hard to see how half a belt could pull the furrow in.  Ironically, the presence of aligned actin filaments in the cleavage furrow was first described in a jellyfish embryo, but it remains unknown how this mechanism works: does something prevent furrow ingression on the far side?  Or is there a belt of connecting material that just doesn’t get focussed enough to ingress?  Or what?

Nanomia is an occasional summer visitor in the plankton in Friday Harbor; in favorable sunlight, the animal looks like a necklace that someone dropped overboard into the sea.  It is a chain of specialized modules, including swimming bells and feeding and reproductive parts, and they spawn unusually-large eggs for a hydrozoan: this cell is slightly compressed, but uncompressed it would have been over 300 µm in diameter.

— text by George von Dassow